Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels.
FIG. 1 depicts a prior art ion implanter system 100. As is typical for most ion implanter systems, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards a target wafer 120 (located in a wafer plane 12). A number of measurement devices, such as a dose control Faraday cup 118, a traveling Faraday cup 124, and a setup Faraday cup 122, may be used to monitor and control the ion beam conditions.
Indirectly heated cathode (IHC) ion sources are typically used in high current ion implantation equipment. FIG. 2 shows a typical IHC ion source 200 in an ion implanter. The ion source 200 comprises an arc chamber 202 with conductive chamber walls 214. At one end of the arc chamber 202 there is a cathode 206 having a tungsten filament 204 located therein. The tungsten filament 204 is coupled to a first power supply 208 capable of supplying a high current. The high current may heat the tungsten filament 204 to cause thermionic emission of electrons. A second power supply 210 may bias the cathode 206 at a much higher potential than the tungsten filament 204 to cause the emitted electrons to accelerate to the cathode and so heat up the cathode 206. The heated cathode 206 may then emit electrons into the arc chamber 202. A third power supply 212 may bias the chamber walls 214 with respect to the cathode 206 so that the electrons are accelerated at a high energy into the arc chamber. A source magnet (not shown) may create a magnetic field B inside the arc chamber 202 to confine the energetic electrons, and a repeller 216 at the other end of the arc chamber 202 may be biased at a same or similar potential as the cathode 206 to repel the energetic electrons. A gas source 218 may supply a reactive species (e.g., GeF4) into the arc chamber 202. The energetic electrons may interact with the reactive species to produce a plasma 20. An extraction electrode (not shown) may then extract ions 22 from the plasma 20 for use in the ion implanter.
A common cause of failure for IHC ion sources is that some materials accumulate on cathode surfaces during extended ion implantation processes. The deposited materials tend to reduce a thermionic emission rate of source ions from the cathode surfaces. Consequently, desired arc currents cannot be obtained and IHC ion sources may have to be replaced in order to maintain normal source operation. The performance degradation and short lifetime of IHC ion sources greatly reduce the productivity of ion implanters.
The above-described problems are especially significant for, but are not limited to, germanium ion implantation. Germanium ion implants have been widely used in the semiconductor industry to pre-amorphize silicon wafers in order to prevent channeling effects. The demand for these pre-amorphizing implants is expected to increase greatly in future semiconductor device manufacturing. One of the most popular source gases for germanium ion beams is germanium fluoride (GeF4) due to its stable chemical properties and cost-effectiveness. However, very short lifetimes of IHC ion sources have been observed while operating with GeF4 gas.
The short lifetime of an IHC ion source used in GeF4 ion implantation may be attributed to excessive, free fluorine atoms in the arc chamber as a result of chemical dissociation of GeF4 molecules. Specifically, arc chamber material may be etched away in chemical reactions with the fluorine atoms, and then some of the arc chamber material may eventually deposit on the cathode resulting in the degradation of electron emissions from the cathode surface.
Other source gases, such as boron fluoride (BF3) and phosphorous fluoride (PH3), may be employed in ion implantation and may cause similar lifetime shortening of IHC ion sources by the stripping (sputtering) of cathode material.
In view of the foregoing, it would be desirable to provide a solution for improving the performance and extending the lifetime of IHC ion sources which overcomes the above-described inadequacies and shortcomings.